ANATOMY OF THE HEART AND BLOOD-VESSELS. 225 



Arterial and Venous Blood. The blood when flowing in 

 tne pulmonary capillaries gives up carbon dioxide to the air 

 and receives oxygen from it; and since its coloring matter 

 (haemoglobin) forms a scarlet compound with oxygen, it flows 

 to the left auricle through the pulmonary veins of a bright 

 red color. This color it maintains until it reaches the sys- 

 temic capillaries, but in these it loses much oxygen to the 

 surrounding tissues and gains much carbon dioxide from them. 

 But the blood coloring-matter which has lost its oxygen has a 

 dark purple color, and since this unoxidized or "reduced" 

 hemoglobin is now in excess, the blood returns to the heart 

 by the venae cavae of a dark purple-red color. This hue it 

 keeps until it reaches the lungs, when the reduced haemoglo- 

 bin becomes again oxidized. The bright red blood, rich in 

 oxygen and poor in carbon dioxide, .is known as "arterial 

 blood" and the dark red as "venous blood:" and it must be 

 borne in mind that the terms have this peculiar technical 

 meaning, and that the pulmonary veins contain arterial blood, 

 and the pulmonary arteries, , venous blood; the change from 

 arterial to venous taking place in the systemic capillaries, and 

 from venous to arterial in the pulmonary capillaries. The 

 chambers of the heart and the great vessels containing arte- 

 rial blood are shaded red in Figs. 90 and 91. 



The Structure of the Arteries. A large artery can by 

 careful dissection be separated into three coats : an internal, 

 a middle, and an outer. The internal coat tears readily across 

 the long axis of the artery and consists of an inner lining of 

 flattened nucleated cells, enveloped by a variable number of 

 layers composed of membranes or networks of elastic tissue. 

 The middle coat is made up of alternating layers of elastic 

 fibres and plain muscular tissue; the former running for the 

 most part longitudinally and the latter across the long axis 

 of the vessel. The outer coat is the toughest and strongest 

 because it is mainly made up of white fibrous connective 

 tissue; it contains a considerable amount of elastic tissue also, 

 and gradually shades off into a loose areolar tissue which 

 forms the sheath of the artery, or the tunica adventitia, and 

 packs it between surrounding parts. The smaller arteries 

 have all the elastic elements less developed. The internal 

 coat is consequently thinner, and the middle coat is made up 

 mainly of involuntary muscular fibres. As a result the large 

 arteries are hiorhly elastic, the aorta being physically much 

 like a piece of india-rubber tubing, while the smaller arte- 



